1. Field of the Invention
The present invention relates to a solid image pickup apparatus having a pixel string in which photoelectric converting sections such as photo diodes are linearly arranged, and more particularly, it relates to the layout configuration of the solid image pickup apparatus.
2. Related Background Art
A CCD linear image sensor for use in an image reader such as a scanner is provided with photoelectric converting sections consisted of photo diodes and the like arranged in strings, and a CCD register. A signal charge photoelectrically converted by each photoelectric converting section is successively transferred to an output section through the CCD register.
Moreover, a color CCD linear image sensor in which three linear sensors are arranged in parallel and reads out different color images, respectively is broadly used in a color reader such as a color scanner.
FIG. 1 is a plan view of a conventional CCD color linear image sensor, and FIG. 2 is an enlarged view showing the constitution in the periphery of the end of a pixel string 1b disposed in the middle of FIG. 1. The image sensor of FIG. 1 is provided with pixel strings 1a, 1b, 1c in which a plurality of photosensitive pixels are arranged in three strings, and color filters (not shown) of red, green, blue and the like are formed on each of the pixel strings 1a, 1b, 1c. The signal charges photoelectrically converted in the respective pixel strings are transferred to CCD registers 3a, 3b, 3c via shift electrodes 2a, 2b, 2c, and they successively move in the CCD registers 3a, 3b, 3c along the arrow directions of FIG. 1. The electric charges reaching the ends of the CCD registers 3a, 3b, 3c are transferred to output circuits 4a, 4b, 4c. The output circuits 4a, 4b, 4c are disposed corresponding to the respective pixel strings 1a, 1b, 1c, and for example, color image signals for RGB are outputted from the output circuits 4a, 4b, 4c. 
In the conventional image sensor shown in FIG. 1, the narrower the distance between the pixel strings becomes, the more the unevenness of resolution is reduced, so that it is possible to simplify the configuration of an optical system for focusing an object light on the light receiving surface of the photo diodes constituting the pixel strings, and the structure of a scanning mechanism system for allowing the image sensor to scan. Therefore, in recent years, the image sensor in which the distance between the pixel strings is narrowed has been vigorously developed.
However, when the distance between the pixel strings is narrowed, the distances between the pixel strings 1a, 1b, 1c and the CCD registers 3a, 3b, 3c are narrowed, so that the amount of a light which leaks from the light receiving surfaces of the respective pixel strings 1a, 1b, 1c to the CCD registers 3a, 3b, 3c increases. Accordingly, there is a problem that owing to the leaked light, excessive electric charges are generated in the CCD registers 3a, 3b, 3c. 
FIG. 3 is a sectional view along the line A—A of FIG. 2. As shown in FIG. 3, a polysilicon layer is formed on a semiconductor substrate via an insulating film 5, and the polysilicon layer is used to form the shift electrodes 2a, 2b, 2c and transfer electrodes 31 to 34 of the CCD registers 3a, 3b, 3c. 
A wiring layer 6 formed of a conductive material such as aluminum is connected to the shift electrodes 2a, 2b, 2c via a contact 7, and voltages for transferring the signal charges are supplied to the shift electrodes 2a, 2b, 2c via the wiring layer 6.
Wiring layers 8, 80 formed of a conductive material such as aluminum are connected to the transfer electrodes 31 to 34 of the CCD registers 3a, 3b, 3c via a contact 9, and voltages for the transfer of the signal charge are supplied from the wiring layers 8, 80 to the CCD registers 3a, 3b, 3c. 
Furthermore, a wiring layer 10 of aluminum is disposed on each of the CCD registers 3a, 3b, 3c, so that no external light enters through the gaps of the wiring layers 6, 8, 80.
The signal charges generated in the pixel strings 1a, 1b, 1c are transferred to the CCD registers 3a, 3b, 3c via the shift electrodes 2a, 2b, 2c, and then, they is transferred in the CCD registers 3a, 3b, 3c along the arrow directions of FIG. 2.
However, in the conventional image sensor, the external light passes through paths shown by the dotted arrows of FIG. 3 and enters a channel area 11 under the shift electrodes 2a, 2b, 2c and an electric charge transfer area 12 of the CCD registers 3a, 3b, 3c, and there is a fear that an S/N ratio is deteriorated.